Japanese Patent Application Laid-open No. 2002-76356 discloses a technique relating to a thin-film transistor (TFT) in the channel layer of which a transparent conductive oxide polycrystalline thin film is used which makes use of ZnO as its chief component. It is noted that this thin film can be formed at a low temperature and is transparent to visible light and hence a flexible transparent TFT can be fabricated on a substrate such as a plastic sheet or a film.
International Publication WO 2005/088726 A1 Pamphlet and Nature, 488, Vol. 432 (2004) also disclose a technique in which a transparent amorphous oxide semiconductor film composed of indium, gallium, zinc and oxygen, (a-IGZO film) is used in the channel layer of a TFT. It is further disclosed that a flexible and transparent TFT showing a good electric field effect mobility of 6 to 9 cm2V−1s−1 can be fabricated at room temperature on a substrate such as a polyethylene terephthalate (PET) film.
Nikkei Microdevice, page 73, FIG. 7, the February issue, 2006, further discloses use of SiON in the insulating layer and inter-element isolation region of a thin-film transistor in the channel layer of which a-IGZO is used.
Including amorphous silicon thin-film transistors (TFTs), TFTs are commonly fabricated through many fine-processing steps. For the purpose of fabrication at low costs of TFTs which can enjoy stable operation, it is important to simplify such fine-processing steps.
In the case when the transparent semiconductor film containing zinc and oxygen is used in the channel region of a TFT, as disclosed in the above Japanese Patent Application Laid-open No. 2002-76356, International Publication WO 2005/088726 A1 Pamphlet and Nature, 488, Vol. 432 (2004), there involves the following difficulty.
The conductive transparent oxide channel region is formed by photolithography and by dry etching or wet etching. The dry etching is usually carried out using an expensive vacuum system, and this is a factor of an increase in fabrication cost. The wet etching is effective in view of cost reduction. In the wet etching, however, there are a case in which device size is limited because of, e.g., a low fine-processing precision, and adsorption of water content to the channel region due to wet processing, and a case in which a low through-put may result because a drying step must be added.